1. Field
Apparatus and methods described in this document relate to electronic circuit designs. More specifically, the apparatus and methods relate to chopper-stabilized analog-to-digital converters and sigma-delta modulators.
2. Background
Sigma-delta modulators and other types of analog-to-digital converters (ADCs) are widely used in communication systems and in many other applications. In communication systems, for example, such devices may be used to convert an intermediate frequency (IF) signal to a baseband frequency signal, in order to meet dynamic range requirements for digital signal processing, and to provide improved adaptability and programmability to receiver circuitry. The advantages of sigma-delta modulators include high dynamic range, which is often needed to distinguish a small desired signal from blockers and interferers. Sigma-delta modulators shape the quantization noise out-of-band, and allow combining decimation filtering with selective digital filtering and IF mixing to attenuate both the quantization noise and neighboring blockers. In addition, the ability to select different sampling rates in the sigma-delta modulator architecture allows a single device to be adapted to different requirements, for example, the requirements imposed by multiple RF standards. Moreover, the use of higher sampling rates and complex digital signal processing allow sigma-delta converters to exhibit relatively low sensitivity to interfering analog signals.
One-over-f (1/f) noise is one type of noise at the output of a typical converter (which word herein applies to sigma-delta modulators and ADCs). One-over-f noise is difficult to reduce because it lies in the same band as the converted signal, making it difficult to filter the noise out of the converted signal. Generally, reducing such noise is costly in terms of chip area and power consumption.
In chopper-stabilized converter topologies, input and output polarities are simultaneously switched in one or more of the converter integrators (generally the first integrator from the input). Because the input and output polarities are switched at about the same time, the combined polarity of the integrator and of the converter are unaffected. The switching of the input and output polarities in the one or more integrators (so-called “chopping”) translates the processed signal inside the operational amplifier (op amp) of the integrator to the higher frequency with which chopping is performed. If the chopping frequency is sufficiently high, the 1/f noise is translated well above the low frequency (e.g., above the audio frequency of less than 20 KHz) of the processed signal. It is easier then to filter out the 1/f noise translated to the higher frequency.
An interested reader may wish to examine Early, U.S. Pat. No. 4,939,516, for more information on chopper-stabilized converters. Another source is YuQing Yang et al. A 114-DB 68-MW CHOPPER-STABILIZED STEREO MULTIBIT AUDIO ADC IN 5.62 MM2, 38 IEEE Journal of Solid-State Circuits 12, at pp. 2061-68 (IEEE 2003).
Chopping is typically done in the middle of the clock phase of the sampling clock of the converter. It takes some time for things to settle down after switching, and if settling time is long compared to the sampling clock period, settling problems may lead to settling distortion. The upper limit on the settling time is thus imposed by the sampling frequency, which is a function of oversampling ratio. Because higher oversampling ratios improve overall system signal-to-noise ratio (SNR), oversampling ratio is a system requirement and is preferably maintained at a relatively high number. Settling time may also be improved by certain design techniques, but such techniques are generally costly from power consumption perspective.
Size, power consumption, speed, SNR, and production costs loom large in designing electronic equipment, and especially portable battery-operated electronic equipment such as wireless access terminals. Therefore, there is a need in the art to improve SNR and reduce the power consumption of electronic equipment, including sigma-delta modulators and ADCs. There is also a need in the art to decrease size and weight of electronic equipment, including sigma-delta modulators and ADCs. There is a further need in the art to lower the cost of manufacturing electronic equipment, including sigma-delta modulators and other devices that include integrators and summers.